An approach to increase the density of an optical disk suggests that an optical disk apparatus using an optical head of which light collection system having a high numerical aperture is configured by a combination of an objective lens and a solid immersion lens (hereafter called “SIL”).
In this system (hereafter called “SIL system”), a material having a high refractive index (about 1.8 to 2.0) is used for the SIL and a protective layer of the optical disk, and gap control to maintain the gap length at a constant value is performed. As a result, the gap length between the SIL and the protective layer of the optical disk becomes a micro value of about 25 nm, and the SIL becomes very close to the protective layer of the optical disk. Information is recorded and reproduced by evanescent light, which is an output light from the SIL obtained in this way, and many documents related to this technique have already been disclosed.
For example, Patent Document 1 discloses an example of the gap control described above.
According to this SIL system, a numerical aperture of the light collecting system, including the SIL, is 1.70 to 1.80, which is approximately double the numerical aperture of a Blu-ray® (hereafter called “BD”) system, which is 0.85. In this case, if the wavelength of the laser is the same, the spot size of the light beam formed on the surface of the information layer of the optical disk becomes approximately half that of the BD system, which means that the recording capacity per unit area, that is the recording density, can be improved to four times that of the BD system.
FIG. 8 is a schematic diagram to described this gap control, and depicts a configuration of a conventional optical disk apparatus that includes the SIL, and FIG. 9 is an enlarged view of the portion P circled (broken line) in FIG. 8.
In FIG. 8, 101 denotes an optical disk, 102 denotes an SIL and 103 denotes an objective lens, where the SIL 102 and the objective lens 103 are integrated by a fixture 104. GL denotes a gap length between the optical disk 101 and the SIL 102, and 107 denotes an actuator for gap control.
An output light of a laser 114 to be a light source is transformed into a parallel light by a collimate lens 111, and becomes a light beam (outgoing) 120 via a polarization beam splitter 109. The light beam 120 is irradiated onto the optical disk 101 via a λ/4 plate 112, the objective lens 103 and the SIL 102. In this case, most of the light beam 120 is reflected by a flat surface 115 of the SIL 102, and becomes a light beam (returning) 121, since the numerical aperture of the light collecting system including the SIL 102 is large, 1.70 to 1.80, as mentioned above, and the rest of the light becomes evanescent light 116 and reaches the recording/reproducing surface of the optical disk 101, as shown in FIG. 9.
The light beam 121 reflected on the flat surface 115 of the SIL 102 enters the polarization beam splitter 109 again via the λ/4 plate 112, and enters a detector 110, which is a gap detection unit, by the function of the polarization beam splitter 109. The detector 110 outputs a gap detection signal GD, and a gap control circuit 113 generates a gap control signal GC by appropriately processing the gap detection signal GD, and drives the actuator for gap control 107. Thus gap control is established.
The λ/4 plate 112 is installed because the return light 121 to the laser 114 can be shielded by disposing the polarization beam splitter 109 between the λ/4 plate 112 and the laser 114.
Now the reason why the detector 110 becomes the gap detection unit will be described.
FIG. 10 is a graph showing a relationship between a gap length GL and the reflected light quantity on the flat surface 115 of the SIL 102 when the effective refractive index of the SIL 102 shown in FIG. 8 is 2.1, and the reflectance of the optical disk 101 is 0.1, where the reflected light quantity in the ordinate is a value normalized by the reflected light quantity when the gap length GL is infinity, and the gap length in the abscissa is a value normalized by the wavelength of the laser 114.
For example, if the wavelength of the laser 114 is 400 nm, then the value 0.05 of the gap length GL shown in the abscissa in FIG. 10 indicates a gap length of 20 nm as an absolute value.
According to FIG. 10, as the value of the gap length GL changes, the reflected light quantity on the flat surface 115 of the SIL 102 changes, therefore the incident power to the detector 110 shown in FIG. 8 also changes, and the detector 110 can function as the gap detection unit.
In other words, in the gap control of an optical disk apparatus including an optical head which uses the SIL 102 as the light collection system, gap detection is performed based on the characteristics of the reflected light quantity from the flat surface 115 of the SIL 102, which changes depending on the gap length GL, as shown in FIG. 10.
In FIG. 10, the reflected light quantity is 0.1 when the gap length GL is zero, that is, when the flat surface 105 of the SIL 102 is in complete contact with the optical disk 101, because the reflectance of the optical disk 101 is assumed to be 0.1.
However the reflected light quantity from the flat surface 115 of the SIL 102 changes depending not only on the change of the gap length GL, but also on the fluctuation of the power of the light beam 120 outputted from the laser 114 in FIG. 8, that is on the laser power fluctuation. In other words, the fluctuation of power of the light beam 120 may be regarded as the fluctuation of the gap, even if the gap did not actually fluctuate, because of the function of the gap detection unit, and gap control is performed based on this fluctuation, hence stability of gap control is markedly diminished.
In order to prevent the fluctuation of power of the light beam 120 outputted from the laser 114, laser power control is performed on the laser 114.
FIG. 11 is a diagram depicting a configuration of a conventional signal recording apparatus which performs gap control including laser power control. The signal recording apparatus shown in FIG. 11 comprises: an information source 201; a recording signal generator 202; an acousto-optical modulator (AOM) 203; a laser element 204 which is a light source to output a laser beam for recording LB1; an electro-optical modulator (EOM) 205; an analyzer 206; a beam splitter (BS) 207; a first photodetector (PD1) 208a; a second photodetector (PD2) 208b; an auto power controller (APC) 209; a first mirror 210a; a second mirror 201b; a third mirror 210c; a first collective lens 211a; a second collective lens 211b; a collimator lens 212; a polarization beam splitter (PBS) 213; a λ/4 plate 214; and an optical head 216 configured by a two-group lens, that is a collective lens 216a and an SIL 216b installed in a piezoelectric element 215.
The signal recording apparatus shown in FIG. 11 also includes: a band limiting circuit 220 that limits the frequency band of return light LB2 from the optical head 216, and outputs return light quantity after the band limitation; and a gap controller 221 that outputs gap control voltage according to the return light quantity after the band limitation outputted from the band limiting circuit 220, so that the distance between the optical head 216 and the glass master disk 217 is controlled to be constant.
In the signal recording apparatus shown in FIG. 11, the laser beam for recording LB1 which entered the first photodetector 208a is converted into an electric signal, and output voltage thereof is inputted into an auto power controller (APC) 209. A difference between this output voltage and a reference voltage is fed back as applied voltage to the EOM 205, whereby the laser power of the laser beam for recording LB1 outputted from the laser element 204 is controlled to be constant.
The composing elements and the functions thereof related to gap control, other than the laser power control described above, are basically the same as the conventional optical disk apparatus shown in FIG. 8, therefore a detailed description here is omitted.
By performing the laser power control shown in FIG. 11, laser power fluctuation of relatively low frequency components due to the temperature characteristics of the laser element 204 can be suppressed, and the negative influence of this power function, that is reflected light quantity fluctuation on the stability of gap control, can be eliminated.
However laser power fluctuation of frequency components, other than relatively low components due to temperature characteristics of the laser element 204, cannot be suppressed, hence if total reflected light quantity fluctuation, that is not due to fluctuation of the gap, such as laser power fluctuation due to a factor other than the temperature characteristics of the laser element 204, is generated, an unrelated light quantity fluctuation, not due to the fluctuation of the gap, is generated, and gap control becomes unstable.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-319160